1. Field of the Invention
The present invention relates to an image sensor unit, an image reader and an image forming apparatus. More specifically, the present invention relates to an image sensor unit used for a scanner, a copier, a multifunction printer and the like, an image reader and an image forming apparatus that employ this image sensor unit.
2. Description of the Related Art
A light guide type illuminating device has been known as an illuminating device used for an image sensor unit.
For instance, Patent Document 1 discloses a conventional technique, which is a contact image sensor unit accommodating, in a frame or the like, a sensor board on which photoelectric converting sensor chips are mounted, an LED light source, a light guide that guides light from the LED light source in the main scanning direction to emit the light in a desired direction.
Unfortunately, the configuration of the unit described in Patent Document 1 requires a dedicated light source in conformity with the position and shape of the light guide, in order to provide the light source on an end face in the longitudinal direction. This causes a problem of increasing the cost.
Thus, image sensor units, for instance, as disclosed in Patent Documents 2 to 4 are disclosed. More specifically, the contact image sensor unit described in Patent Document 2 includes a light guide that is substantially rod-shaped and has end portions bent at right angles toward a sensor board, and an LED irradiates a light-incident surface of the light guide (a light-incident surface opposite to the sensor board) with light. A reflective surface is formed at a flexion of the light guide. Light incident on the incident surface is reflected by the reflective surface, thereby being guided into a portion at which a diffusing surface and an emission surface are formed. Accordingly, the light entering into the light guide is diffused by the diffusing surface and emitted from the emission surface to an external sheet. Light reflected by the sheet travels in parallel to the direction in which the LED emits light, and reaches a sensor unit. The contact image sensor unit described in Patent Document 3 includes a substantially rod-shaped a light guide formed such that a main region and an auxiliary region are connected to each other in series along the longitudinal direction, and a light source irradiating the auxiliary region of the light guide with light. The auxiliary region is for allowing the light from the light source to travel into the main region, and includes a downward facing light-incident surface that receives the light from the light source, and a reflective surface that reflects the incident light toward the main region. Furthermore, a reflector, which covers portions other than the incident surface, is provided in the auxiliary region, thereby preventing the light from leaking from the portions other than the incident surface. This configuration enables illumination efficiency at a reading line of a sheet to be improved. Patent Document 4 discloses an example of an arrangement of an LED light source on a sensor board, and a positional relationship of a light-incident surface of a light guide opposed to a light emitting surface of the LED light source.    Patent Document 1: Japanese Laid-open Patent Publication No. 11-331494    Patent Document 2: Japanese Laid-open Patent Publication No. 10-93765    Patent Document 3: International Publication Pamphlet No. WO2004/054232    Patent Document 4: Japanese Laid-open Patent Publication No. 2004-193773
On scanners, copiers and multifunction printers there are increasing demands for reduction in size, weight and cost. Accordingly, also on an image sensor unit incorporated therein, there are further demands for reduction in size, weight and cost. For the sake of reduction in size, weight and cost of the image sensor unit, it is typically required to use an inexpensive surface-mounted light source while reducing the size and weight of a light source and a circuit board on which image sensor unit is mounted. Unfortunately, according to the configuration described in Patent Document 2 or 3, the light source, such as LED, is required to be arranged immediately below an incident surface of the light guide. This causes a problem in that it is difficult to reduce the size and weight of the circuit board and, in turn, difficult to reduce the size and weight of the image sensor unit.